Multiple sclerosis (MS) is one of most common neurological diseases and has a high tendency to attack young adults. The progression of the disease is associated with multiple episodes separated in time and anatomic spaces in the central nervous system (CNS). Current research and therapy for MS is challenged by the lack of an efficient and specific method for the early assessment of an autoimmune response in the evolution of MS. The diagnosis of MS, based on the standard MS diagnostic criteria, is usually made only after the disorder has progressed significantly and long after the onset inciting events. Previously, we have provided the first evidence of an antigen-driven selected B cell clonal expansion in the CNS from the majority of MS patients, indicating an autoimmune process in the pathogenesis of MS. Additional preliminary research, B cell clonal expansion can be detected in the early stage of the disease to plaques from chronic progressive MS (indicating secondary progressive MS and primary progressive MS). Therefore, the primary goal of the present proposal is to determine the significance and pathobiology of B cell clonal expansion in the CNS of MS. Three specific aims/hypotheses will be addressed using analyses of B cell clonality and somatic mutations on the genes of immunoglobulin (Ig), previously shown to evidence the process an antigen-driven selected immune response in the pathogenesis of MS. The experiments will involve the following strategies: 1) Analyze B cell clonality of CSF and plaque cells to confirm whether B cell clonal expansion in the CNS is common feature of MS and its clinical course. If this is correct, its predictive value for the development of MS after a first episode with symptoms suggestive of MS will be investigated. 2) The direct role of B cell clonal expansion in the pathogenesis of lesion formation. B cell clonal expansion and its role in the lesion formation will be examined by analyzing tissue-specific infiltration of clonally expanded CSF B cells and persistent B cell expansion in lesions. 3) Analyze differentiation pathways of B cell clonal expansion to identify the role of antigen driven autoimmunity and natural autoimmunity in the pathogenesis of MS and its clinical course. Our work will provide important new data on the complexity of immune responses in the evolution of MS and afford a means to design early diagnosis and classification for therapeutic planning.